HLCL'98 and EXPRESS98

by Uwe Nestmann and Ilaria.Castellani

EXPRESS98 was the 5th International Workshop on Expressiveness
in Concurrency. These workshops, the first four of which were
held as meetings of the European HCM network EXPRESS (1994-1997),
focused on the comparison of different computational models and
paradigms on the basis of their expressive power. HLCL98, the 3rd International Workshop on High-Level Concurrent
Languages, was intended to bring together active researchers involved
in the design, development, foundations, and applications of high-level
concurrent programming languages and models.

Both events took place this year as pre- and post-satellite workshops
to the 9th International Conference on Concurrency Theory, CONCUR98,
which was organized this year by INRIA Sophia-Antipolis in Nice,
France. Around 60 researchers attended the satellite events.

EXPRESS

The EXPRESS workshops aim at bringing together researchers interested
in the relations between various formal systems, particularly
in the field of Concurrency. More specifically, they focus on
the comparison between programming concepts (such as concurrent,
functional, imperative, logic and object-oriented programming)
and mathematical models of computation (such as process algebras,
Petri nets, event structures, modal logics, rewrite systems etc.)
on the basis of their relative expressive power.

The EXPRESS98 workshop started with an invited talk by Matthew
Hennessy, reviewing some recently proposed location calculi. These
are extensions of name-passing calculi where names may also represent
sites, thus allowing for the modelling of process migration. The
following two presentations focussed on the comparison of the
expressiveness of several name-passing calculi inspired by the
pi-calculus and proposed new mathematical concepts such as that
of extended subset. The second morning session was more eclectic
but not less interesting, devoted to expressiveness issues in
process algebras with data and rule formats for higher-order languages.
After lunch, the participants reconvened for the second invited
talk by P. S. Thiagarajan, who gave an account on the extension
of results from the classical automata theory to a model for distributed
systems. Following two talks on reactive probabilistic processes
and local event structures, there was a session on the relative
expressive power of different dialects of the language LINDA.
Jos Baeten gave a lively concluding talk comparing two known semantics
for process algebras on the basis of their capacity to express
deadlocked behaviours.

HLCL

Programming models should be simple, practical, high-level, and
well founded. These qualities allow rigorous language specifications
and support both formal and informal reasoning about programs.
For concurrent and distributed systems, recent research on programming
models has driven the design of several encouraging programming
languages including Erlang, versions of ML, like CML and Facile
and Haskell, as well as languages explicitly designed for concurrency
or distribution such as Obliq, Oz, Pict, and the Join-Calculus
language. Although the motivations behind the design of these
languages are diverse (ranging from the development of graphical
user interfaces and multi-agent systems to constraint, real-time,
and distributed programming), suitable foundations have turned
out to be quite similar in style and technique, often based on
variants of well-known calculi for mobile processes.

After the invited talk (by James E. White, initiator of Telescript
and chief technology officer at General Magic Inc., USA) with
the provocative title Why networks arent programmable (or, why
programs arent protocols), the program was first devoted to the
foundations of safe distributed programming with mobile agents.
Thereafter, presentations on implementation solutions, static
type systems, and object-oriented and the functional paradigms
in the setting of concurrent programming rounded off the high-quality
programme. Apart from the high standard of the presentations,
much of the success of this workshop was generated by contrasting
the traditionally rather technical audience with an industrialist
invited speaker, which opened up very lively discussions on possible
future developments and expectations, for both the theoretical
and practical aspects of high-level concurrent and distributed
languages.

The proceedings of both events will be published by Elsevier Science
as Numbers 2 and 3 of Volume 16 of Electronic Notes in Theoretical
Computer Science.